1. Field of Invention
The field of invention is neutron detectors. More particularly, the present invention relates to a neutron detector having solid absorbers.
2. Description of the Related Art
Neutron detectors are useful in many industries. One industry, for example, in which neutron detectors are widely used is the oil industry, in which neutron detectors are used to detect potential oil yielding sites. Oil producing formations deep in the earth emit neutrons at a different rate than water bearing formations or non-fluid bearing rock. A device using neutron detection for logging oil wells is disclosed in U.S. Pat. No. 4,641,028 issued to Taylor et al., on Feb. 3, 1987.
The '028 patent teaches a well logging instrument for use in a cased well bore. The '028 device contains a sealed source of fast neutrons and two identical thermal neutron detectors with a volume of four atmospheres of helium-3 gas. The formation surrounding the cased well bore is bombarded with high energy neutrons and the two thermal neutron detectors are spaced apart from one another and from the source to receive slowed down or thermal neutrons from the surrounding formations. The epithermal or fast neutrons striking the formation are slowed down by fluids containing great quantities of hydrogen or chlorine atoms, creating thermal or slow neutrons, which the detectors respond to logarithmically and independently. The counting rate of each detector is processed independently to count rate meters and to a recorder to present two outputs of information. The presence or absence of hydrogen and chlorine atoms in the formations as detected by each detector.
In U.S. Pat. No. 5,532,482 issued to Stephenson on Jul. 2, 1996, teaches a method for determining a characteristic of an underground formation. The '482 method includes the steps of irradiating the formation with high-energy neutrons and detecting neutrons scattered by the formation. The detected neutrons have energies above epithermal to determine the nature of the formation matrix. Stephenson teaches that epithermal neutrons can also be detected to determine formation porosity. Stephenson utilizes an apparatus having a high-energy neutron source, typically a D-T accelerator producing 14 MeV neutrons, and detectors such as 4He-filled proportional counters for detecting neutrons having energies above epithermal and 3He-filled proportional counters for detecting epithermal neutrons.
In addition to the oil industry, neutron detectors are also commonly used in the medical industry. Neutron detectors are also useful for surveillance in nuclear facilities and weapons storage. While several specific utilities of neutron detectors are mentioned, it is well known to those skilled in the neutron detection art that neutron detectors are useful in many applications.
Neutrons are uncharged particles that can travel through matter without ionizing the matter. Because neutrons travel through matter in such a manner, neutrons are difficult to detect directly. Some other evidence of a neutron event must be detected in order to determine its existence. An indirect method detects the result of a neutron event and not the neutron event itself.
The use of indirect detection of neutrons is known in the art. For example, a neutron detector as disclosed in U.S. Pat. No. 5,334,840 issued to Newacheck et al., on Aug. 2, 1994, detects photons of light emitted by carbon infiltrated boron nitride in its hexagonal form when the compound is bombarded by neutrons. The amount of light detected correlates to the number of neutrons bombarding the boron nitride.
Another neutron detector commercially available utilizes helium-3 as the neutron absorber, such as in the '028 device described above. When bombarded by neutrons, helium-3 decomposes into H and H3 having combined kinetic energies of 764 keV. The ionization of the gas electrons can be detected using conventional methods well known in the art and further described below. This type of neutron detector requires a long collection time for the resulting ionization requiring integrating and differentiating time constants of between 1 and 5 microseconds for the best results.
Other gas mixtures are commercially available that have varying resolution or charge per pulse yields depending on the gases used.
Neutron detection for monitoring the dose of thermal neutrons given to patients receiving boron neutron-capture therapy has used lithium-6 and a cerium activator in a glass fiber. See M. Bliss et al., “Real-Time Dosimetry for Boron Neutron-Capture Therapy”, IEEE Trans. Nucl. Sci., Vol. 42, No. 4, 639-43 (1995). Hiller et al., in U.S. Pat. No. 5,973,328, issued on Oct. 26, 1999, improve this technique by allowing a cerium-activated glass fiber to be coated with fissionable elements. A wet chemistry method of placing radioactive fissile elements into glass—which in the vitrified state does not pose a hazard—as described in the '328 patent using sol-gel based technology, is a significant benefit. M. Ghioni et al., “Compact Active Quenching Circuit for Fast Photon Counting with Avalanche Photodiodes”, Rev. Sci. Instr., 67, 3440-48 (1996), describe an avalanche photodiode implementation for detecting neutron induced ionization and optical pulse detection.
The '328 device introduced sol-gel techniques unique in the art of neutron detection. Sol-gel chemistry was first discovered in the late 1800s. This area of chemistry has received renewed interest when the process was found useful in producing monolithic inorganic gels at low temperatures that could be converted to glasses without a high temperature melting process. C. J. Brinker et al., “Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing” (Academic Press, Inc., New York 1990) provide a comprehensive explanation of sol-gel chemistry. Sheng Dai et al., “Spectroscopic Investigation of the Photochemistry of Uranyl-Doped Sol-Gel Glasses Immersed in Ethanol”, Inorg. Chem., 35, 7786-90 (1996), provide further detail disclosing uranyl-doped sol-gel glasses.
Emissions detectors such as microchannel plates, channeltrons, and avalanche photodiodes are in common use for detecting ultraviolet (UV) light and fissioned charged particles such as electrons or protons. Microchannel plates are commercially available and well known in the art. Typically a microchannel plate is formed from lead glass having a uniform porous structure of millions of tiny holes or microchannels. Each microchannel functions as a channel electron multiplier, relatively independent of adjacent channels. A thin metal electrode is vacuum deposited on both the input and output surfaces to electrically connect channels in parallel. Microchannel plates can be assembled in stacked series to enhance gain and performance.
The microchannel plates serve to amplify emissions from fissionable material resulting from the bombardment of neutrons. The amplified signal is then detected and recorded. The signal frequency is proportional to the charged particle emissions, which are proportional to the amount of neutrons bombarding the fissionable material.
Channeltrons operate on the same basic principal of amplifying proportional signals emitted from fissionable materials. A channeltron is a horn-shaped continuous dynode structure that is coated on the inside with an electron emissive material. An ion striking the channeltron creates secondary electrons that have an avalanche effect to create more secondary electrons and finally a current pulse.
Typically due to the exotic materials and sensitivity of the equipment, the neutron detectors currently available are expensive and difficult to maintain. For example helium-3 is an extremely rare stable isotope and must be separated at considerable expense from the radioactive gas tritium. Furthermore, the use of a gas absorber results in a slower response time than a solid absorber as disclosed herein. The '328 device thus incorporated fissionable material into a sol-gel composition in combination with an emission detector.
Neutron scattering is a powerful tool for conducting scientific studies of the physical geometry of molecules important in biology and material science. Protein structure and the structure of superconductors are of immediate practical importance. The detection of the scattered neutrons is an area where advancements are necessary. Specifically of concern are the ability to locate the scattered neutrons and the ability to rapidly process the neutron signals as the detection rate becomes very rapid.
Currently, a major research facility using spallation is being constructed by the Department of Energy. Spallation is a nuclear reaction in which incident particles bombard an atomic nucleus to eject particles from the nucleus. The Spallation Neutron Source (SNS) facility under construction in Oak Ridge, Tenn., is designed to have an output of pulsed neutrons that is the most intense in the world of its type. The SNS is provided with multiple experimental stations using pulsed neutrons. One such experimental station locates the neutrons diffracted from a target on an x-y plane. The time of the neutron absorption on the x-y plane surface is then referenced to a fiducial timing signal to an accuracy of within 100 nanoseconds of the absorption event.
Mori et al., “Measurement of Neutron and γ-ray Intensity Distributions with an Optical Fiber-Scintillator Detector,” Nuclear Instruments and Methods in Physics Research, A 422, 129-132 (1999), describe a ZnS(Ag) scintillator with lithium-6 on the tip of an optical fiber for locating neutrons with a position resolution of 1 mm in a 10 minute interrogation and within a volume wherein the tip is extendable one meter. Gorin et al., “Development of Scintillation Imaging Device for Cold Neutrons,” Nuclear Instruments and Methods in Physics Research, A 479, 456-460 (2002), have described using ZnS(Ag) and lithium-6 fluoride coupled to wavelength-shifting fibers. Gorin et al., employ arrays of fibers in two planes rotated ninety degrees relative to each other such that an absorbed neutron is located in the plane to a resolution of 1 mm.
Wallace et al., Nuclear Instruments and Methods A 483 (2002) 764-773 report the gamma insensitivity of the thin film lithiated glass. This specificity of the lithiated glass for generating a signal in the presence of gamma radiation has application to the monitoring of spent nuclear fuel rods and for the determination of the fissile mass within remote handle transuranic waste.
Other methods and devices have been developed for neutron detection. Typical of the art are those methods and devices disclosed in the following U.S. Patents:
Pat. No.Inventor(s)Issue Date3,222,521K. EinfeldDec. 7, 19654,365,159C. A. YoungDec. 21, 19824,481,421C. A. Young et al.Nov. 6, 19845,002,721P. Bernard et al.Mar. 26, 19915,216,249B. L. Jones et al.Jun. 1, 19935,289,510J. T. MihalczoFeb. 22, 19945,336,889K. J. HofstetterAug. 9, 19945,345,084R. C. ByrdSept. 6, 19945,656,815B. L. Justus et al.Aug. 12, 19975,659,177R. L. Schulte et al.Aug. 19, 19975,726,453R. G. Lott et al.Mar. 10, 19985,880,471J. Schelton et al.Mar. 9, 19996,134,289A. J. Peurrung et al.Oct. 17, 2000
Of these patents, Einfeld ('521) teaches a method and apparatus for the non-destructive testing of a substance to determine the concentration of two or more fissionable isotopes in the substance. Einfeld teaches generation of first and second neutron spectrums, each having a unique mean energy. Determination of the number of fissions as a function of the neutron spectrum applied follows from the counting of the prompt and/or delayed neutrons produced by the fissions.
Young ('159) teaches a neutron detection apparatus including a selected number of flat surfaces of lithium-6 foil. A gas mixture is in contact with each of the flat surfaces for selectively reacting to charged particles emitted by or radiated from the lithium foil. A container is provided to seal the lithium foil and the gas mixture in a volume from which water vapor and atmospheric gases are excluded, the container having one or more walls capable of transmitting neutrons. Monitoring equipment in contact with the gas mixture detects reactions taking place in the gas mixture, and, in response to such reactions, provides notice of the flux of neutrons passing through the volume of the detector.
Similarly, Young et al., ('421) teach a neutron detection apparatus is provided including a selected number of surfaces of lithium-6 coated wire mesh in contact with a gas mixture for selectively reacting to charged particles emitted or radiated by the lithium-6 coated mesh. As in the '159 device, a container is provided to seal the lithium-6 coated mesh and the gas mixture in a volume from which water vapor and atmospheric gases are excluded, the container having one or more walls capable of transmitting to neutrons. Monitoring equipment in contact with the gas mixture detects the generation of charged particles in the gas mixture and, in response to such charged particles, provides an indication of the flux of neutrons passing through the volume of the detector.
Bernard et al., ('721) disclose a first and second assemblies. The first assembly includes a neutron radiation source, a hydrogenated material block having a recess for receiving the neutron radiation source, and a first neutron detector. The second assembly includes a hydrogenated material block surrounded by a material stopping the thermal neutrons and a second neutron detector placed in the hydrogenated material block. The first and second assemblies are placed on either side of the area to be controlled.
Jones et al., ('249) disclose a neutron detector formed from a layer of polycrystalline diamond deposited by a chemical vapor deposition process. The diamond material contains 10B as a dopant, in a concentration of 1,000 ppm or less. In one embodiment, the layer of diamond material is deposited on a supporting substrate, which may comprise a layer of diamond material. In another embodiment, the substrate comprises a layer of conductive or semi-conductive material. The diamond material is deposited on top of the substrate, with at least two regions in contact with the semi-conductive material. A central portion of the semi-conductive material is etched away, and electrical contacts are applied to the remaining portions of the semi-conductive substrate. A stated advantage of the '249 neutron detector is that the diamond layer is very thin, less than 50 μm thick, so that the sensitivity of the detector to other radiation, particularly γ radiation, is reduced.
In the '510 patent, Mihalzco teaches nuclear reaction detectors capable of position sensitivity with sub-millimeter resolution in two dimensions. The nuclear reaction detectors include two arrays of scintillation or wavelength shifting optical fibers. Each array is formed of a plurality of optical fibers disposed in a side-by-side relationship. The two arrays are disposed in X- and Y-directions with respect to each other, with a layer of nuclear reactive material disposed between and operatively associated with surface regions of the optical fiber arrays. Each nuclear reaction occurring in the layer of nuclear reactive material produces energetic particles for simultaneously providing a light pulse in a single optical fiber in the X-oriented array and in a single optical fiber in the Y-oriented array. These pulses of light are transmitted to a signal producing circuit for providing signals indicative of the X-Y coordinates of each nuclear event.
The nuclear reactive material of the '510 patent is doped with a phosphor such as calcium tungstate, magnesium tungstate, zinc silicate, zinc sulfide, cadmium tungstate, and cadmium borate. Mihalzco further teaches that a compound such as 6LiF or glass or plastic scintillators containing 235U, 10B, or 238U provides a concentration of the phosphor dopant in the layer of nuclear reactive material sufficient to assure that an adequate distribution of phosphor to be contacted by and react with the energetic particle produced from each nuclear reaction. Normally, a concentration of the phosphor dopant in the range of about 100 ppm to about 2 percent by volume is adequate for the purposes of the Mihalzco detectors.
Hofstetter ('889) discloses a gamma radiation detector using a radioluminescent composition. The detector includes a radioluminescent composition that emits light in a characteristic wavelength region when exposed to γ radiation. The composition contains a scintillant such as anglesite (PbSO4) or cerussite (PbCO3) incorporated into an inert, porous glass matrix via a sol-gel process. Particles of radiation-sensitive scintillant are added to a sol solution. The mixture is polymerized to form a gel, then dried under conditions that preserve the structural integrity and radiation sensitivity of the scintillant. The final product is a composition containing the uniformly-dispersed scintillant in an inert, optically transparent and highly porous matrix. Hofstetter describes the resulting composition as chemically inert and substantially impervious to environmental conditions.
In the Byrd ('084) device, a plurality of omnidirectional radiation detectors is arranged in a close packed symmetrical pattern to form a segmented detector. The output radiation counts from these detectors are arithmetically combined to provide the direction of a source of incident radiation. Output counts from paired detectors are subtracted to yield a vector direction toward the radiation source. The counts from all of the detectors are combined to yield an output signal functionally related to the radiation source strength.
Justus et al., ('815) teach an apparatus and method for detecting radiation using a light transparent thermoluminescent dosimeter. The dosimeter includes a nanocrystalline phosphor dispersed in a light transparent glassy matrix. The dosimeter is placed in an environment in which radiation is present for a period of time, removed from the environment and then heated to emit light. The intensity of the light emission is indicative of the radiation dose. The method is used to detect and record the dosage of both UV and ionizing radiation. In one embodiment of the '815 device in which the dosimeter is in the form of a flat plate, ambient α or β radiation populates traps. The flat dosimeter is then heated and the emitted light is guided to the edges of the dosimeter by total internal reflection, where it is detected and recorded.
R. L. Schulte et al., ('177) teach a directional thermal neutron detector for detecting thermal neutrons and determining the direction of the thermal neutron source. The directional detector includes an array of individual thermal neutron detector modules, each of which comprises front and back planar silicon detectors between which is disposed a gadolinium foil. The array comprises a plurality of individual detector modules angularly displaced with respect to each other. The direction of the thermal neutron source is determined by comparing the magnitudes of the output signals from the plurality of angularly displaced detector modules. Each thermal neutron detector module is segmented into four quadrants to reduce its capacitance and resultant noise. The thickness of the gadolinium foil in each thermal neutron detector module is at least 15 microns thick, to improve the front-to-back silicon detector counting ratio to ascertain the side (front or back) from which thermal neutrons are arriving at the detector. The thick gadolinium foil makes each detector module substantially opaque to thermal neutrons, and the detector modules are positioned relative to each other in the array to shield one another from thermal neutrons, thereby enhancing the angular resolution of the directional thermal neutron detector. Gamma rays are discriminated against by using coincidence signal processing within the elements of the detector sandwich to reduce the gamma ray contribution to the total signal.
Lott et al., ('453) disclose a radiation resistant solid state neutron detector. The '453 detector uses a neutron converter material such as boron or lithium to react with neutrons to create charged particles that are received in a semiconductor active region of the detector. The active thickness of the detector is smaller than the range of the charged particles. Since most of the radiation damage produced by impinging charged particles occurs near the end of the range of the particles, displacement damage predominantly occurs outside of the active region. Although the charged particles pass through the semiconductor material, the particles cause electron excitation within the semiconductor material, the electron excitation being detected in the form of an electronic pulse. The '453 detector is provided to increase resistance to radiation damage, improve high temperature operation, and to obtain real time measurements of neutron flux in reactor cavities and other previously inaccessible locations.
Schelton et al., ('471) disclose a neutron detector for the detection of thermal neutrons. The '471 neutron detector includes 6LiF layers for the conversion of the neutrons to ionizing radiation. The 6LiF layers are surrounded by layers for detecting the ionizing radiation generated by the neutrons in the 6LiF layers.
Peurrung et al., ('289) teach a system for measuring a thermal neutron emission from a neutron source. The '289 device includes a reflector/moderator proximate the neutron source that reflects and moderates neutrons from the neutron source. The reflector/moderator further directs thermal neutrons toward an unmoderated thermal neutron detector.
Other references of interest in the art of neutron detection include:                H. Krinninger et al., “Pulsed Neutron Method for Non-Destructive and Simultaneous Determination of the 235U and 239Pu Contents of Irradiated and Non-Irradiated Reactor Fuel Elements,” Nucl. Instr. Meth. 73, 13-33 (1969);        M. Zanarini et al., “Evaluation of Hydrogen Content in Metallic Samples by Neutron Computed Tomography,” IEEE Trans. Nucl. Sci., 42, 580-84 (1995);        C. M. Logan et al., “Observed Penetration of 14-MeV Neutrons in Various Materials,” Nucl. Sci. Eng. 115, 38-42 (1993);        H. Jaeger et al., “Two-Detector Coincidence Routing Circuit for Personal Computer-Based Multichannel Analyzer,” Rev. Sci. Instrum. 66, 3069-70 (1995);        E. J. T. Burns et al., “A Solenoidal and Monocusp Ion Source (SAMIS),” Rev. Sci. Instr., 67, 1657-60 (1996);        S. T. Coyle et al., “A Low Cost Preamplifier for Fast Pulses From Microchannel Plates,” Rev. Sci. Instr., 66 4000-01 (1995); and        Y. G. Kudenko et al., “Extruded Plastic Counters with WLS Fiber Readout,” Nucl. Inst. And Meth. A 469, 340-346 (2001).        